It has slightly come back to me:
subst Q: "C:\Documents and Settings\Username\My Documents"
Would map Q: to to the above path.
So you could say put in variables to the below but for simplicity I'll make it easy:
copy x:\roms\*.* c:\snesroms /y
subst z: c:\snesroms
Then whilst in the rom browser z: would be mapped to the local browser.
To delete a path it's subst z: (or driveletter) /d
Once it's been done it doesn't disappear either so it could be useful?